This invention relates to a method for making separation membranes from poly(phenylene sulfide) resins.
Poly(phenylene sulfide) (PPS) polymers are known to have excellent thermal stability and chemical resistance. For these reasons, it has been attempted to make membranes from these polymers.
Membranes are often made by extruding a solution of a polymer in a suitable solvent. With most polymers, this technique is readily applied because the polymers form high viscosity solutions which can be extruded to form membranes having useful properties. However, it has been found to be difficult to prepare membranes in this manner from poly(phenylene sulfide). Unless the polymer concentration is very high, PPS solutions have viscosities so low that they do not maintain their shape as, for example, sheet, tubules or hollow fibers, after they are extruded. These low viscosity solutions are also difficult to feed and convey using the typical screw extrusion equipment since this equipment is designed to use the melt viscosity of the extrudate to provide a pumping action. For these reasons, it is difficult to extrude these solutions into a membrane. As a result, the membrane is either impossible to form, or contains imperfections such as pinholes which destroy the membrane's ability to perform good separations.
By increasing the PPS concentration, the viscosity of PPS solutions can be increased enough to extrude them. However, membranes made from highly concentrated PPS have very low porosity. The fluxes of these membranes are therefore too low to be suitable for commercial use.
It it therefore desirable to provide a method by which poly(phenylene sulfide) polymers can be formed into membranes which are substantially free of imperfections and which exhibit fluxes which are high enough to be useful commercially.